Poly-ethylene-terephtalate (PET) is a favorable substrate for flexible electronic devices and trace membranes (Vilenskey et al., 2002; Toufik et al., 2002). PET is also well suited for many large area electronic applications, such as, displays and simple circuits (Gao at el., 1997; Ragsten et al., 1996). PET is a tough material that is chemically resistant to many solvents and cannot be easily dissolved to form membranes for many device applications. PET can tolerate temperatures as high as 120° C. although short thermal annealing at temperatures up to 150° C. has little effect on the physical characteristics of the substrate (Dreuth at el., 1999). PET may also be used as a track membrane acting as a molecular sieve for filtration (Vilenskey, 2002; Nazmov, 2001). Ultra-violet radiation has been used to accelerate the etching process (Apel, 2001).
Microstructure elements have dimensions in the micron range and production of these structures is important in precision engineering, mircooptics, microelectronics, micromechanics, and others. It is well known in the art that these microstructures may be produced from plastic, metal, or ceramic by the process of lithography, electroforming, and casting. Despite the advantages offered by the lithography process simpler processes are often desired. In the PET etching methods previously reported, irradiation by X-ray or bombardment by energetic ions are first used to create latent tracks followed by a chemical etching process (Vilenskey, 2002; Nazmov, 2001; Steckenreiter, 1995). Apart from these techniques, laser ablation and dry etching are used for the processing of PET substrates (Roberts, 1997; Dadsetan, 1999; Iwanishi, 2001; Rossier, 2002). The laser micro-machining is expensive and requires complex facilities.
Thermocouple devices are among the most frequently used sensors in many applications (Wagner, 1990). In the case of polysilicon-based thermocouple devices, the processing temperature exceeds 600° C. where many substrates such as PET cannot withstand. Metallic-based thermocouple devices require much lower temperatures during processing and fabrication of thermocouples can be added on thin rubber membranes and flexible bases (Fortunato, 2000). However, the low sensitivity of the junction makes the readout circuitry rather complex. Germanium can be processed at temperatures far below that of silicon and the junction sensitivity remains high. In addition, silicon-germanium alloys may be suitable for making high performance thermocouple based devices (Van Gerwen, 1996). Although realization of these high sensitivity thermocouples is possible on PET, the overall device performance requires formation of thin membranes on a plastic substrate.